ZSM-5 crystalline aluminosilicate zeolites and methods for preparing ZMS-5 are known. For example. U.S. Pat. No. 3,702,886, the entire contents of which are incorporated herein by reference, discloses a process for preparing ZSM-5 using a mixture of alkali metal cations and tetraalkylammonium cations, such as tetrapropylammonium (TPA) cations. In this process, the tetraalkylammonium cations are believed to act as a template to direct the synthesis of the ZSM-5 structure so that the resultant zeolite contains the template within its pore structure.
European Patent No. EP-A-21674 teaches that large crystal ZSM-5, having a crystal size in excess of 1 micron, can be prepared from a reaction mixture containing TPA cations, provided that the OH—/SiO2 ratio is maintained within the range from 0.01-0.07.
Relatively large crystal ZSM-5 (>1 micron) usually can be prepared without difficulty when the aluminum content in the synthesis mixture is low. As a result, with large crystal ZSM-5, although SiO2/Al2O3 molar ratios of 100 or higher are relatively easily attainable, SiO2/Al2O3 molar ratios of 40 or lower are typically rather difficult to attain.
Catalysts prepared from ZMS-5 crystals of medium (about 0.5 micron in size) and especially large (greater than about 1 micron in size) size and high activity (low SiO2/Al2O3 molar ratio) can be useful in, for example, selective toluene disproportionation to produce para-xylene, an aromatic raw material that can be used to make polyethylene terephthalate (PET).
U.S. Pat. No. 6,013,239, the entire contents of which are incorporated herein by reference, discloses a method of preparing large crystal ZSM-5 (of the order of 3 microns) with high aluminum content (SiO2/Al2O3=25-40), but only in the presence of amino acids. Examples of suitable amino acids are said to include 6-aminohexanoic acid, N-2-adamantylglycine, N-cyclohexylglycine, lysine, and glutamic acid (and its monosodium salt). The amino acids are believed to act as nucleation suppressing agents rather than templates or structure directing agents, since the amino acids do not appear to become entrained in the pore structure of the resultant zeolite. However, attempts to scale-up this process have to date proved unsuccessful, since, due to the high viscosity of the synthesis gel, it has not been possible to reproduce the large crystal size materials in relatively larger scale batches.
According to the invention, it has now been found that by using relatively low surface area (<150 in/g) silica sources in combination with specific nucleation suppressors in the synthesis gel, it is possible to produce ZSM-5 having a relatively large crystal size (≧about 2 microns) and a relatively high alumina content (SiO2/Al2O3 molar ratio of 40 or lower). In addition, the reaction gels produced using these new formulations have been found to be functional at lower viscosities than those employing conventional silica sources, making the process more readily amenable to larger scale syntheses.
U.S. Pat. No. 4,088,605 discloses a method of synthesizing a crystalline aluminosilicate zeolite, such as ZSM-5, having an aluminum-free outer shell of SiO2, said zeolite being made by a two stage method comprising (i) initiating crystallization in a crystallization medium to produce the zeolite, and (ii) then altering the crystallization medium to substantially eliminate the aluminum therein, for example, by adding a complexing agent for the aluminum selected from the group consisting of gluconic acid, tartaric acid, nitrilotriacetic acid and ethylenediaminetetraacetic acid. There is no disclosure or suggestion in the '605 patent that, by adding the aluminum complexing agent at the initiation of the synthesis, it would be possible to produce large crystal ZSM-5 with a uniformly high aluminum content, and hence activity.